Carbohydrates

Question 1

Describe a biochemical test to show a solutions contains a non-reducing sugar. [3]

Answer 1

1) heat w a dilute acid [e.g. HCl] to hydrolyse the disaccharides into monosaccharides + neutralise [e.g. sodium hydrogen carbonate]
2) heat w Benedict’s solution
3) red ppt. forms if a high conc of non-reducing sugars present [yellow/green if low conc]

Question 2

Suggest a method, other than using a colorimeter, to measure the quantity of reducing sugar in a solution after carrying out the Benedict’s test. [2]

Answer 2

1) filter + dry the ppt. [using a paper towel]
2) weigh the mass [using a balance]

Question 3

Why would the use of a colorimeter improve the repeatability when carrying out the Benedict’s test? [1]

Answer 3

1) colour change is subjective so converts colour into a quantitative measure
2) standardises the method

Question 4

Describe the differences between the structure of a cellulose molecule + a glycogen molecule. [4]

Answer 4

1) cellulose is made up of β-glucose monomers + glycogen is made up of α-glucose monomers
2) cellulose molecule has a straight chain + glycogen is branched
3) cellulose molecule has a straight chain + glycogen is coiled
4) glycogen has 1,4- + 1,6- glycosidic bonds + cellulose only has 1,4- glycosidic bonds

Question 5

What features of starch make it a good storage molecule. [5]

Answer 5

1) insoluble in water, so doesn’t affect water potential
2) branched/coiled, so molecule compact + can fit many molecules in small area
3) polymer of α-glucose so provides glucose for respiration
4) branched so more ends for enzyme action so is hydrolysed quickly
5) large molecule so can’t cross cell membrane

Question 6

Describe the structure of glycogen. [2]

Answer 6

1) polysaccharide/polymer of α-glucose
2) joined by glycosidic bonds
3) branched structure

Question 7

Explain how glycogen is adapted for its function. [4]

Answer 7

1) branched more ends for enzyme action so hydrolyses quickly to form glucose
2) which is used in respiration
3) insoluble, so doesn’t affect water potential
4) branched/coiled, so molecule compact + can fit many molecules in small area

Question 8

What monomers make up:
1) maltose
2) lactose
3) sucrose
[3]

Answer 8

1) glucose + glucose
2) glucose + galactose
3) glucose + fructose

Question 9

Explain how cellulose molecules are adapted for the function in plant cells. [3]

Answer 9

1) long + straight chains that run parallel
2) cross linked by many hydrogen bonds to form fibrils
3) provides strength/support/rigidity to cell wall
4) prevents plant cell walls from bursting as water enters by osmosis as exerts an inward pressure stopping any further influx of water

Question 10

What are the 2 polysaccharides of α-glucose that starch is made from?

Answer 10

• amylose
• amylopectin

Question 11

Describe the features of amylose.

Answer 11

• unbranched chain of α-glucose
• joined by 1,4 glycosidic bonds
• coiled so v. compact: stores lots of energy as fits lots of molecules in small area

Question 12

Describe the features of amylopectin:

Answer 12

• branched chain of α-glucose joined by 1,4 + 1,6 glycosidic bonds
• lots of branch ends for enzyme activity so can be hydrolysed quickly to release glucose

Question 13

How do you test for reducing sugars?

Answer 13

• add food sample to test tube
• heat w Benedict’s Reagent
• brick red (high conc), yellow/green (low conc) = positive

Question 14

Describe the biochemical test for starch.

Answer 14

• add iodine dissolved in potassium iodine solution
• turns blue-black if starch is present